1. Electrical activity in beta-cells of mouse islets of Langerhans was studied using ultra-fine micro-electrodes. 11.1 mM glucose stimulated electrical activity in bursts of 12 to 24 sec duration, including an active phase, characterized by depolarization and rapid fluctuations of the potential (spikes), and a silent phase when the membrane repolarized. Analysis of the spike frequency distribution during the active phase supports the hypothesis that the spikes result from exocytosis. 2. Calcium removal, in the presence of magnesium, did not alter membrane potential. Burst activity in 11.1 mM glucose was inhibited, and spike activity appeared in some cells. The frequency of these spikes increased as the magnesium was reduced. Re-introduction of calcium induced depolarization and increased spike frequency. 3. Omission of calcium and magnesium reversibly induced depolarization. In 11.1 mM glucose, there was further depolarization, and an altered pattern of electrical activity. Bursts were reversed in polarity, the silent phase being depolarized with respect to the active phase, and duration of reversed bursts was increased while the number of spikes during the active phase was decreased. Caffeine, when added, increased the number of spikes. Re-introduction of calcium and magnesium induced a transient hyperpolarization with suppression of spike activity. 4. Simultaneous reduction of calcium and magnesium to one-half or one-tenth of the control concentration enhanced the overall spike activity in 11.1 mM glucose by shortening or blocking the silent phase between bursts. 5. Quinidine, 0.14 mM, irreversibly inhibited the silent phases of bursts and modified the biphasic electrical response to glucose.